1. Field of the Invention
This invention relates to thermally powered terahertz (THz) and infrared (IR) radiation sources and more specifically to the integration of thermally powered low-dimensional nano-scale oscillators such as nanowires and nanotubes that emit radiation with micro-scale resonant defect cavities in photonic crystals to efficiently select, collect and transmit the emitted radiation in a specified narrow band.
2. Description of the Related Art
THz-frequency radiation, in the frequency region from 300 GHz to 10 THz, has been relatively unexploited compared to the adjacent radio frequency (RF) and IR spectral bands. This is largely because of transmission difficulties due to absorption by atmospheric water vapor but also due to a lack of practical radiation sources. In recent years there has been a significant growth of interest in applications of this previously underutilized portion of the electromagnetic spectrum. These applications include active short range imaging systems for concealed weapon detection or driving aids in dust or sand storms. The shorter wavelengths provide higher image resolution than is possible with traditional radar systems operating at radio frequencies. At longer ranges, the THz band is very useful for wide bandwidth space-based communications, high-resolution imaging of rotating satellites from another space-based platform and other space object surveillance applications. The lack of atmospheric attenuation at high altitudes permits use of THz radiation.
Spectroscopy is another application area for THz radiation. Many biological agents have abundant and easily recognized resonances in the THz region. From simple content analysis (material identification by exciting, then detecting molecular vibrational and rotation states) to spectroscopic imaging of trace clouds of biological agents, the THz spectrum promises to open many applications in the bio-detection area. Mounting a THz system on an unmanned aerial vehicle may make it possible to detect and map biological and certain chemical warfare agents on a battlefield. Researchers in the U.K. recently reported that THz radiation is 100% successful in detecting skin cancer, but they don't yet understand how (Scott, W. B., Potential applications of terahertz signals spur scientists to explore RF/light border region, Aviation Week & Space Technology, Jun. 21, 2004, page 68). Passive THz systems have also been used in astronomy to map molecular clouds in the galaxy.
One of the major bottlenecks for the successful implementation of THz-frequency systems is the limited output power of conventional THz sources. Most systems produce THz radiation via optical techniques, but those require massive lasers, complex optical networks and cooling systems. Some of the THz sources reported in the literature include optically pumped THz lasers, time-domain spectroscopy, backward wave oscillators, solid-state amplifiers combined with direct multipliers, and photo-mixers (Iida, M. et al., Enhanced generation of terahertz radiation using 3D photonic crystals with a planar defect, Proc. CLEO/QELS, June 2002 (Baltimore), Section CM1; Unterrainer, K. et al.; Cavity enhanced few cycle THz generation and coherent spectroscopy, Proc. CLEO/QELS, June 2002 (Baltimore), Section CM1; Han, H., Park, H., Cho, M., and Kim, J., Terahertz pulse propagation in a plastic photonic crystal fiber, Applied Physics Lett., 80 #15, 15 Apr. 2002). The different sources have disadvantages including limited output power; excessive cost, size and weight; poor reliability and limited frequency agility.
Three dimensional solid objects emit electromagnetic radiation in a spectral distribution which is described by the Planck equation for any given temperature of the object. This thermal spectral distribution is broad and continuous and peaks in the infrared (IR) band for room temperature objects, with surface emissivity variations providing the only deviation from the Planck distribution.
It is well known that an object's Planck blackbody radiation spectrum may be strongly modified when the object is a photonic crystal with a band gap positioned around the peak (e.g., 5-10 μm) of the Planck spectrum. Several theoretical and experimental papers in this area have been published with very interesting results including Zhi-Yuan Li, Phys. Rev. B 66, R241 103 (2002) and S-Y. Lin, et al, Phys. Rev. B 62, R2243 (2000).
U.S. Pat. No. 7,078,697 recites the use of photonic crystals to shift the thermal emission peak associated with the standard Planck blackbody spectral distribution from the IR band to the THz region. The photonic crystal core is designed to shift the photon density of states (DOS) such that the thermal radiation from the broad IR Planck peak shifts to a sharp peak in the THz region (0.3-10 THz). The photonic crystal core is combined with a waveguide and power combining structure so that the radiated THz energy is efficiently collected and directed to an output antenna. More specifically, a plurality of defect cavities can be formed in the photonic crystal layer to collect and concentrate the radiation in a limited bandwidth. The defect cavities then couple the radiation to an adjacent waveguide that directs the radiation to the output antenna.
Theoretical studies of one dimensional objects indicate a less smooth spectral distribution of thermal radiation from the objects. The one dimensional structures (compared to the radiation wavelengths involved) have restricted vibrational modes that channel more thermal energy into useful frequencies. Chan et al. “Thermal emission and design in one-dimensional periodic metallic photonic crystal slabs”, Physical Review E 74, Jul. 26, 2006 presents a framework for understanding the physical phenomena that drive thermal emission in one-dimensional periodic metallic photonic crystals. A. M. Nemilentsau et al “Thermal Radiation From Carbon Nanotube in Terahertz Range” Physics Rev. Lett. 99, 147403 Oct. 5, 2007 theoretically investigates the thermal radiation from an isolated finite-length carbon nanotube (CNT) both in near- and far-field zones. The formation of the discrete spectrum (See FIG. 1(b)) in metallic CNTs in the terahertz range is demonstrated due to the reflection of strongly slowed-down surface-plasmon modes from CNT ends.